This research will test the hypothesis that the functional development of the chick cochlea depends upon the gradual acquisition of a sharply-tuned electrical resonance in the cochlear hair cells. Further, this electrical resonance arises from the action of time- and voltage-dependent ionic conductances in the hair cell membrane. Thus, the onset of cochlear function will be correlated with the appearance of those channels which underlie the resonance. By describing the cellular basis of functional development, this investigation will add to our understanding of the cochlear filter mechanism, and so improve our ability to diagnose auditory dysfunction. Further, it is hoped that by examining the details of hair cell development it may prove possible to better understand how membrane differentiation is controlled, especially the possible role of electrical activity in that process. The resonance properties of individual hair cells will be described by recording the intracellular voltage change produced in hair cells by injection of current through the microelectrode. The damped voltage oscillation or ringing that follows a voltage transient in electrically tuned cells is characteristic of the particular resonance giving rise to it. Thus, the filtering capability, or tuning, of the hair cell can be defined electrically. Since cochlear function in the chick gradually improves from embryonic day 15 through two weeks posthatch, the ringing behavior of hair cells in the embryonic cochlea will be recorded to determine if electrical tuning develops concurrently. Whole cell current and single channel current will be recorded from single hair cells dissociated from the embryonic cochlea. Here the aim will be to find if some ionic current, and more specifically, a particular type of ionic channel, appears in the hair cell at the time that sharp tuning develops. Finally, with a view toward developing a preparation in which hair cell differentiation can be experimentally manipulated, in vitro development of the chick otocyst will be studied. The undifferentiated otic epithelium in a 4-day chick embryo can be maintained in tissue culture for several weeks. Under the appropriate conditions, hair cells and supporting cells will differentiate. It will be of considerable interest to ask whether these cells are electrically tuned and, if so, how their tuning might be directed.